I don't see why it should be an option to the PC [to join and/or explore the Enclave] ~not any more than they should have the option to join an ant colony.
what if the PC IS from the Enclave ranks though? :3 like, a clone created with pure DNA strands as part of an experiment?
That's the same as saying, "What if the PC is an ant, born in the hive". Ants will only accept other ants from their hive, and are only concerned with their hive... Enclave would only accept other Enclave [from their hive], and are only concerned with their own agenda. It's then the reverse... The Enclave PC would [deservingly] be shunned and mistrusted by all~not of the Enclave; unless the experiment went rogue, and they defected; in which case they would not be accepted by the Enclave ~anymore. You can't really have public ties with an offending army, and that army's oppressed, and be trusted and equally liked by both.
The Enclave PC would be indoctrinated, and view the rest of the world's population as an infestation of mutants ~~or not, but if not, then they have strayed from the Enclave beliefs, and would no longer be accepted among them. Such a game IMO, is batter suited as an Enclave themed FO:Tactics clone, than a Fallout sequel. Bethesda could pull it off in a FO5 of course, but then [it goes without saying] they are not bound by plausibility or any sense of responsibility to respect the setting and/or make it practical or believable ~even within the series context. Basically it would get the standard alternate player race treatment ~ala TES... It would be Fallout's Khajiit and Argonians; [Ghouls/Enclave] ~and a disservice IMO, because they wouldn't follow through with it. It would be merely cosmetic, and ultimately unsatisfying I think.